In a point-to-multipoint wireless communication system, a base station (BS) communicates with multiple customer premise equipments (CPEs) or user terminals. The data transmission from the BS to a CPE is called the downstream (DS) and the data transmission from a CPE to the BS is called the upstream (US).
Orthogonal frequency division multiple access (OFDMA) is a popular multiple access approach widely used in wireless communication systems including 802.16 WiMAX and 802.22 WRAN. OFDMA uses orthogonal frequency multiplexing (OFDM) as the modulation method. In OFDM the channel bandwidth in the frequency domain is divided into multiple orthogonal narrow band channels (subcarriers). The sharing of transmission resources in OFDMA among multiple CPEs is two dimensional: a subcarrier can be assigned to a CPE for a number of symbols, and each CPE can be assigned one or more modules (bursts) in the subcarrier by symbol plane (i.e., in the time-frequency two-dimensional plane). In the DS, preambles are used at the beginning or middle of the symbol transmissions. These preambles serve purposes such as synchronization, channel estimation and correction. Values known to both the transmitter and the receivers are also transmitted at specific pilot subcarriers to provide better channel tracking and correction capabilities. In the US, a preamble is not transmitted by a CPE to the BS. The BS usually relies on the pilots transmitted by a CPE to estimate the channel.